As the electromagnetic instrument industry has been advanced, various display devices have been developed. Also, image apparatuses, computers, mobile communication terminals and the like in which the display devices are used trend to be developed. A liquid crystal display (LCD) device appeared by this trend is being spotlighted as a display device for a monitor and a mobile communication terminal and the like.
A liquid crystal display (LCD) device applies the electrical and optical properties of liquid crystal with an intermediate property between liquid and solid to a display device, and is a electrical device which changes and transmits various electrical information generated from various kinds of devices to visual information using a variation in transmittance of the liquid crystal depending on an approved voltage. The liquid crystal display device is a flat display device which has been widely used because power consumption is small due to a low operating voltage, and it is convenient to carry about it.
Since the LCD device has no self-luminosity which emits light itself, a backlight is always required from all LCD devices. The backlight performs the function of a light source of the LCD device, a composite composed of a power circuit for driving a light source as well as the light source itself for irradiating light to a rear surface of a liquid crystal module and all adjuncts for forming uniform flat light is called a backlight unit (BLU). A backlight unit using a light emitting diode (LED) has been recently suggested as a light source to illuminate the LCD. The LED is a light emitting device which generates light using a light emitting phenomenon generated when a voltage is applied to a semiconductor. This LED is advantageous because it is small-sized compared to a conventional light source and has a long lifespan. Also, it is advantageous that the LED has a low operating voltage with high energy efficiency because electrical energy is directly changed to light energy.
The backlight unit is used in such a manner that a plurality of light source devices such as the LED is mounted on the printed circuit board (PBC). Because the printed circuit board should endure heat emitted from the light source devices, a metal material is mainly used for it. However, if the heat emitted from the light source devices is not properly emitted, the problem that the light source devices are destroyed or the lifespan is reduced is generated.
FIG. 1 is a view illustrating cross sections of a flat printed circuit board and a bent printed circuit board.
Referring to (a) on FIG. 1, a flat printed circuit board 100 includes a metal plate 10, an insulating layer 120, a circuit pattern 130 and a solder-resist 140. The circuit pattern 130 is formed by laminating a metal layer such as a copper layer on the insulating layer 120 and patterning the metal layer. The solder-resist 140 is applied to the circuit pattern 130 to prevent a solder bridge phenomenon between circuits at the time of soldering from being generated, and thus the circuits may be protected. A light source device, namely, an LED, is mounted on the flat printed circuit board 100.
To improve a heat dissipating property, as illustrated in (b) on FIG. 1, the flat printed circuit board 100 may be bent by a bending process so that it becomes a bent printed circuit board 110. The bent printed circuit board 110 may emit a lot of heat compared to the flat printed circuit board. The reason is because the bent printed circuit board 110 may provide a wider radiation area compared to the flat printed circuit board 100 if spaces to be mounted are the same. For example, as illustrated in FIG. 2, the bent printed circuit board is mounted in a backlight unit.
FIG. 2 illustrates a cross-sectional view showing that a bracket 40 and the bend printed circuit board 110 manufactured according to a conventional art are mounted in a chassis 50 which is a light guiding passage of the backlight unit.
Referring to FIG. 1, the bent printed circuit board 110, and the bracket 40 for fixing it to the chassis 50 are separately manufactured, and are bonded to each other using a thermal interface material 20 (TIM). Also, they are mounted in the chassis 50 used as the light guiding passage using a thermal interface material 30. The LED (not drawn) is mounted on the bent printed circuit board 110. The bracket 40 in which the bent printed circuit board 110 is installed also has a similar shape to that of the bent printed circuit board 110.
Like this, the flat printed circuit board for emitting heat well from the LED has an L shape due to a bending process. However, the bent printed circuit board 110 may be varied by a spring back phenomenon.
FIG. 3 is a view for explaining a spring back phenomenon.
Referring to FIG. 3, the spring back phenomenon is called an elastic operation trying to be restored into an original shape when a plastic material is bent by a bending process and a pressure is then removed from it, and a phenomenon that an amount of bending is reduced. The flat printed circuit board 100 suffers the elastic operation trying to be restored into its original flat shape in an arrow direction as illustrated in FIG. 3.
FIG. 4 is a view showing a bent printed circuit board in which a spring back phenomenon is generated.
Referring to FIG. 4, one part of the bent printed circuit board 110 is illustrated. The illustrated part is a flat part of the bent printed circuit board 110. When the bent printed circuit board 10 formed by bending the flat printed circuit board suffers the spring back phenomenon, bowing may be generated at the bent printed circuit board 10. In this case, it was problematic that when an LED mounting part 100 is mounted on the bent printed circuit board 10, adhesion or a coupling property to the bent printed circuit board 10 is reduced due to the bowing of the bent printed circuit board 10, and an optical property of the LED is reduced.